Rosetta’s views of Comet 67P

Mauro Pirarba BSc, a Planetary Sciences Graduate Certificate student and Fellow of the Royal Astronomical Society, discusses Dr Ramy El-Maarry’s recent talk on the geology of Comet 67P/Churyumov-Gerasimenko.

Figure 1 – Five comets have been photographed at close range during flyby missions, but only one, comet 67P, has been studied closely for an extensive period of time (image credit: El-Maarry et al., 2019, https://link.springer.com/article/10.1007%2Fs11214-019-0602-1)

Comets are temperamental, often breaking all rules, suddenly appearing out of nowhere, occasionally getting close enough to the Sun and the Earth to display long tails that make us marvel at their beauty and diversity. They have been studied by astronomers for centuries and yet we still do not understand them fully.

What can Dr. Ramy El-Maarry, a geologist at Birkbeck College, possibly tell about one of them, Comet 67P/Churyumov-Gerasimenko, at a special meeting organized by the Royal Astronomical Society?

Does that C-something – G-something sound vaguely familiar? Perhaps you are more likely to remember another name, Rosetta, the probe of the European Space Agency (ESA) tasked with deciphering its mysteries, a few years ago. Launched in 2004, Rosetta reached Comet 67P in August 2014 and spent two years observing it closely.

“I have 80,000 images from that mission and 20 minutes to go through them…”, Ramy started his talk, to everybody’s laughter.

In fact, such a big figure hints at an unprecedented and extraordinary achievement. Twelve spacecrafts have sent back to Earth data about eight different comets and images of six of them (see figure.1). What makes Rosetta stand out is the length of the observation, two years, and its closeness, on average a few tens of kilometres. You may also remember that the mission included a lander, Philae, which failed to anchor itself to the ground and bounced a few times, before settling down and sending back images and data. The greatest feat though was achieved by the “mothership”, Rosetta, which accompanied the comet for most of its orbit around the Sun, taking images that show details as small as a fraction of a meter. These images have allowed scientists for the first time to observe geological processes, as they happened, on the surface of a comet.

We are all familiar with the effects of water, ice, temperature excursions and wind in weathering, transporting and depositing sediments, reshaping the landscape on the Earth. We’ve seen images of craters and the effects of earthquakes and volcanic eruptions on our planet. What geological processes has ESA’s spacecraft uncovered on Comet Churyumov-Gerasimenko?

This is a tiny world with a miniscule gravity and an irregular shape (approximately 4 x 4 x 2 km), made of porous and light material rich in different ices. The agent driving most geological processes on P67 is solar radiation. As the comet orbits the Sun along a very elongated orbit, which takes it further away from our star than Jupiter and then brings it not much closer to the Sun than Mars, insolation varies dramatically and seasons become extreme. Autumn and winter last about 5.5 years in the northern hemisphere, while the southern spring and summer last nearly a year and are relatively hot, causing the icy surface to sublimate copiously, creating a coma, a tenuous atmosphere. Activity is patchy, occasionally “violent”, jets of gas burst into space taking dust and larger particles with them. Significant amounts of gas and some of the dust are lost, but part of the solid material is transported by “winds” to the northern hemisphere, blanketing it with dust and coarser grains. Over time, the material moves down gentle and steep slopes, “pushed” by the weak gravity, forming a variety of terrains. These give the northern hemisphere a very different look from that of the southern half, which is quite rugged (see figure.2). One of the most striking images shown by Ramy was one where aeolian ripples appeared on an otherwise smooth terrain in the neck of the comet, a narrow region connecting the two main “lumps” that make up 67P. No one had ever thought such features could occur on a comet.

Figure 2 -Dr. El-Maarry shows how different the northern and southern hemispheres of comet 67P appear (image credit: El-Maarry et al., 2019, https://link.springer.com/article/10.1007%2Fs11214-019-0602-1 and El-Maarry et al., 2016, https://www.aanda.org/component/article?access=doi&doi=10.1051/0004-6361/201628634).

The rate of ices’ sublimation is so high in some areas that several meters of materials are removed during the summer, uncovering a variety of features. Some of these are circular and resemble craters, others look like depressions that wax and wane over time. Their origin is baffling geologists, as that of other so-called transient surface features. Somewhere else on the surface of the comet new pits (probably sinkholes) appear and cliffs collapse. Outbursts of activity, driven by the Sun’s heat, propel jets that like rockets push the ground in the opposite direction, varying the speed of rotation of the comet. The resulting forces cause tension, leading to the formation of tectonic fractures, as several photos shown by Dr. El-Maarry clearly prove. Picture after picture a tiny complex word emerges, revealing to us in detail what we had been able to observe only from very far or for very brief instants, in the case of other comets.

In August 2016, Rosetta ended its mission and landed softly on Comet Churyumov-Gerasimenko. The legacy of the mission is not just what it has taught us about Comet P67, but it goes beyond. As Ramy summed up at the end of his talk, the number of images and other data collected in situ is helping scientists not only to understand 67P, but also interpret previous and future mission to other comets. A long time is likely to pass, before a probe like Rosetta will be launched.

In the meantime, Comet Interceptor, a new exciting cometary mission is taking shape at ESA. We’ll ask Ramy to bring a few thousand pictures next time…

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The Search for Life in the Universe

This post was contributed by Phyllis Hughes, editor of U3A magazine Sources. Phyllis attended the Birkbeck Science Week 2016 U3A talk: “The search for life in the universe – The new science of astrobiology” on Monday April 11 at the University of East London.

Science Week - astrobiology event Ian CrawfordThe search for life in the universe ranging from simple organisms to intelligent beings was the subject for Prof Ian Crawford from Birkbeck University.

Prof Crawford is an astrobiologist whose work looks at the possibility of life on other planets. He told his audience of members of the University of the Third Age that Earth was the only place currently where life was known to exist.

Scientists therefore were concentrating their research on planets that were thought to have been similar to Earth when life first developed.

“Life appeared on Earth fairly soon after the planet formed,” he said. “However it took a long time for microbiological life to develop into multi-cell animals.”

He said that Mars was currently of interest because it was known that it had an atmosphere similar to Earth 4bn years ago when simple life forms were first developing. This atmosphere contained water and was comparatively warm.

In 1976 the Viking space probes took samples of the Mars soil to see if there was evidence of life, but these had proved negative. However the samples were very small and research was continuing in this field.

Professor Ian Crawford

Professor Ian Crawford

Other places that were thought to be worth investigating included one of the moons of Jupiter, called Europa which was first discovered by Galileo in 1609.

Two moons of Saturn, Enceladus and Titan were also potentially similar in atmosphere.

As well as examining the geology there was also interest in trying to pick up radio signals from the galaxy that might have been transmitted by intelligent life. The Search for Extra Terrestial Intelligence (SETI) had also proved negative so far.

“Given all the necessary factors I think the possibility of advanced technological life is rare,” Prof Crawford said.

Find out more

View the full Science Week 2016 programme of free events

Courses at the School of Science

Prof Ian Crawford

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Out of this World: An Evening with the Planets

This post was contributed by Henry Rummins, Communications Manager at Birkbeck, University of London

1.-SpaceIt is a question which humans have pondered for thousands of years when looking up at the night sky and seeing the thousands of dots of light which gradually twinkle into view: what is out there, beyond our world?

It was these questions, too, which led Dr Louise Alexander, now a post-Doctoral researcher at the UCL/Birkbeck Centre for Planetary Sciences, to follow her curiosity and begin a journey which would begin at introductory classes at UCL to a Master’s degree and PhD at Birkbeck to her current destination, analysing rock samples brought back from the Apollo 12 mission to the moon, to determine lunar geology.

Her story was one of six presented on the evening in a showcase of planetary wonders hosted by Steve Cross, Head of Public Engagement at UCL, comedian and founder of Science Showoff, looking at aspects of the solar system ranging from our closest neighbour, the moon, to distant Pluto and beyond. Yet while the planets were the stars of the show, the stories of all the researchers in planetary science reminded us that reflecting on the cosmos often brings that questioning back down to Earth, and what it means to be human; to look up and the night sky, and wonder.

It was a theme which ran through Clara Sousa Silva’s look at Twinkle, a space mission that will analyse light reflected from planets outside the solar system to reveal the chemical composition of their atmospheres, as well as, it’s hoped, their weather and history, giving crucial clues to worlds outside the solar system and potentially spotting clues for life elsewhere.

As well as the science behind the project, she also looked at how to encourage more women into studying science in school through to university level, and subsequently pursing research as a career option, illustrating her point with some sobering facts on the current low level of female participation in the sector.

A fly-by of planetary science made up the evening’s contribution from Dr Pete Grindrod, who busted some of the most widely believed myths about Mars, with a look at the origins of planets and the Rosetta comet mission by Geraint Jones and a look at missions to Jupiter by Lucia Ray making up the trio.

The evening rounded off with the first performance of a new piece of music interpreting the celestial dance between Pluto and its moon Charon, called Pluto and Charon – A Planetary Waltz. The piece – commissioned by the Centre for Planetary Sciences at UCL/Birkbeck – was accompanied by grainy footage of Charon orbiting Pluto, enhancing the original piece played as a piano duet between Valentina Pravodelov and Kerry Yong. It ended the evening where we started: stimulating the excitement and curiosity of wondering, what’s out there?

An Evening with the Planets was presented by The Centre for Planetary Sciences at UCL/Birkbeck

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